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Are you referring to this form: https://www.jotform.com/62837945120155?
I checked that form and it looks like you currently have a Purchase Order payment integration in the form. This is why you are receiving the message that you "have already integrated a payment system".
If you want to use Paypal as the method of payment, you will have to delete the Purchase Order payment integration and replace it with the Paypal payment integration. You can follow the screen recording below:
You can also follow this guide for more information about the Paypal payment integration: PayPal Forms. Cheers.